A known hydropneumatic harpoon is disclosed in Bulgarian Inventor's Certificate BG 32725. The known device comprises an arrow, base and body in which is mounted a hydraulic chamber and in the rear end of which is screwed the pneumoaccumulator body. The front of the pneumoaccumulator rests against a rubber membrane with which are sealed the pneumoaccumulator and the hydraulic chamber. The front end of the rubber membrane is set against a discharging grid while the extension is mounted on a mobile barrel. The rear end of the pneumoaccumulator is closed by a bottom. The rear half of the mobile barrel operates at the same time as a cut-off valve, the sealing edge of which rests against a rubber ring. The rubber ring is mounted on the base that is screwed in the front end of the hydraulic chamber. A barrel is also screwed to the base. The seat of the trigger valve is mounted perpendicularly to the longitudinal axis of the discharge grid and is closed under the action of a spring. In the discharge grid there is an opening which connects the rear valve volume with the volume formed over the trigger valve.
A disadvantage of this known hydropneumatic harpoon is that it is manufactured in different lengths depending on the fish dimension for which it is destined since the killing force of the arrow is depending directly on its length. In the harpoon art, three types of harpoon lengths are generally used: small (up to 0.5 m), medium (from 0.7 to 1.4 m) and large (over 1.5 m). From a harpoon with a given length, only an arrow with a fixed diameter can be shot, thus imposing to manufacture items with different arrow diameters depending on conditions under which there are used e.g., sand or stone bottom etc.